1. Technical Field
The present invention relates to a process for preparing a 2,4-dichloro-3-alkyl-6-nitrophenol useful as intermediate of a cyan coupler in color photography.
2. Prior Art
As a process for preparing 2,4-dichloro-3-ethyl-6-nitrophenol, Japanese Patent Laid-Open Publication Nos. 44552/1988 and 47741/1989 disclose a method for obtaining it by nitrating 3,5-dichloro-4-ethylnitrobenzene to give 3,5-dichloro-4-ethyl-1,2-dinitrobenzene and hydrolyzing the nitro group at the 2-position of the dinitrobenzene with an alkali metal hydroxide.
Japanese Patent Laid-Open Publication No. 60634/1986 discloses a method for obtaining a 2,4-dichloro-3-alkyl-6-nitrophenol by chlorinating a p-alkylnitrobenzene with chlorine to give a 2,3,5-trichloro-4-alkylnitrobenzene, which is then subjected to hydrolysis.
Japanese Patent Laid-Open Publication No. 205447/1985 discloses a method for obtaining a 2,4-dichloro-3-ethyl-6-nitrobenzene by chlorinating 5-ethyl-2-nitrophenol with sulfuryl chloride in the presence of a catalyst.
Japanese Patent Laid-Open Publication No. 228943/1989 discloses a method for obtaining 2,4-dichloro-3-ethyl-6-nitrophenol by chlorinating p-ethylbenzenesulfonic acid to give 2,3,5-trichloro-4-ethylbenzenesulfonic acid, desulfonating and then nitrating the 2,3,6-trichloro-ethylbenzene obtained to give 2,3,5-trichloro-4-ethylnitrobenzene, and hydrolyzing a chlorine of the nitrobenzene.
Japanese Patent Laid-Open Publication No. 57536/1986 discloses a method for preparing 2,4-dichloro-3-ethyl-6-nitrophenol by sulfonating 4-chloro-3-ethylphenol or m-ethylphenol with a sulfonic anhydride complex in a halogenated hydrocarbon or with an excessive amount of concentrated sulfuric acid to give 5-chloro-4-ethyl-2-hydroxybenzenesulfonic acid or 4-ethyl-2-hydroxybenzenesulfonic acid and chlorinating the sulfonic acids with chlorine gas or sulfuryl chloride to form 3,5-dichloro-4-ethyl-2-hydroxybenzenesulfonic acid, of which the sulfonic group is then substituted by a nitro group with use of nitric acid.
All of the aforementioned well-known methods have various problems to be solved in order to practice them in industrial scale. For instance, in the method which uses 3,5-dichloro-4-ethyl-nitrobenzene as a starting material, difficulties are encountered in preparation of the starting compound. That is, nitration of ethylbenzene with use of mixed acid gives p-ethylnitrobenzene which is in the form of a mixture of ortho- and para-isomers.
Purification and separation of the p-isomer from the o-isomer is required and yield at this stage is not so high. Moreover, dichlorination of the p-ethylnitrobenzene accompanies undesirable isomers. Accordingly, complicated purification is actually required.
The method of trichlorination of p-ethylnitrobenzene also has the same defects as described above.
The method by using 5-ethyl-2-nitrophenol as a starting material is not practical by reason of the fact that it is difficult to prepare this starting material. That is, nitration of m-ethylphenol produces a mixture of isomers, and that it is required to remove by-products.
In the method with use of p-ethylbenzenesulfonic acid as a starting material, it is also required to separate the p-ethylbenzenesulfonic acid as a starting material from the o- or m-isomers on the preparation of the p-isomer. Such a method, in which chlorination is conducted in a large amount of sulfonic acid, has also problems such as a low volumetric efficiency or the treatment of sulfuric acid.
The method with use of m-ethylphenol as a starting material is not preferred, since it produces isomers in the sulfonation of the m-ethylphenol, so that it gives a low yield. The method, in which 4-chloro-3-ethylphenol is sulfonated with a sulfuric anhydride complex, has problems that special care is required because of the danger in handling of sulfuric anhydride. In addition, a raw material such as dioxane, which is required for the formation of the complex, is difficult to be recovered.
The method, in which 4-chloro-3-ethylphenol is sulfonated by concentrated sulfuric acid, also has problems on its industrialization. That is, volumertic efficiency is low due to the necessity of concentrated sulfuric acid in an excessive amount up to 5-7 moles to the amount of the ethylphenol and the treatment operation of sulfuric acid is difficult.
The method of chlorination with chlorine gas or sulfuryl chloride is disadvantageous for the practice in an industrial scale. When chlorine gas is used, a special safety device is required because of its nature. When sulfuryl chloride is used, the reagent is expensive and the reaction is required to be conducted in an organic solvent.
The present invention provides an industrially advantageous method for preparing a 2,4-dichloro-3-alkyl-6-nitrophenol as a final object, which has fewer problems described above.